MDEV-31948 Add class DBNameBuffer, split check_db_name() into stages

- Adding a class Lex_ident_fs, to store identifiers for on-disk
  database objects, such as databases, tables, triggers.

- Moving the validation code from check_db_name()
  to non-modifying methods in Lex_ident_fs:

    Lex_ident_fs::check_body()
    Lex_ident_fs::check_db_name()

  Adding a new method Lex_ident_fs::check_db_name_with_error(),
  which performs validation and raises an error on validation failure.

  Unlike the old function check_db_name(), the new class Lex_ident_fs
  does not lower-case the identifier during the validation.
  Lower-casing must be done before calling Lex_ident_fs validation methods.

- Adding a low level helper template class CharBuffer which can:
  * store exact or lower-cased strings with a short fixed maximum length
  * return the value as a LEX_CSTRING efficiently

- Adding a helper template class DBNameBuffer (deriving from CharBuffer), to
  allocate optionally lower-cased database identifiers on stack when relevant.
  Useful for temporary values which don't need to be allocated on MEM_ROOT.

- Using DBNameBuffer in mysql_change_db()

- Using DBNameBuffer in show_create_db()

sql/sql_class.h

@@ -24,6 +24,7 @@
 #include "dur_prop.h"
 #include <waiting_threads.h>
 #include "sql_const.h"
+#include "lex_ident.h"
 #include "sql_used.h"
 #include <mysql/plugin_audit.h>
 #include "log.h"